When voltage is applied on an organic electroluminescence device (hereinafter, referred to as an organic EL device), holes and electrons are respectively injected into an emitting layer from an anode and a cathode. The injected electrons and holes are recombined in an emitting layer to form excitons. Here, according to the electron spin statistics theory, singlet excitons and triplet excitons are generated at a ratio of 25%:75%. In the classification according to the emission principle, in a fluorescent EL device which uses emission caused by singlet excitons, the limited value of an internal quantum efficiency of the organic EL device is believed to be 25%. On the other hand, in a phosphorescent EL device which uses emission caused by triplet excitons, it has been known that the internal quantum efficiency can be improved up to 100% when intersystem crossing efficiently occurs from the singlet excitons.
As an example of such an organic EL device with use of fluorescence and phosphorescence, a white-emitting organic EL device, in which a fluorescent-emitting material exhibiting a blue emission, a phosphorescent-emitting material exhibiting a red emission and a phosphorescent-emitting material exhibiting a green emission are used, has been proposed. However, it has been found that a luminous efficiency is low with an arrangement of simply layering a blue fluorescent-emitting layer, a red phosphorescent-emitting layer and a green phosphorescent-emitting layer, so that a study for improving the luminous efficiency has been made.
As a result, it has been found that, when the fluorescent-emitting layer and the phosphorescent-emitting layer are laminated adjacent to each other, holes and electrons are not recombined in the layers in good balance and, further, triplet energy of the phosphorescent-emitting material in the phosphorescent-emitting layer is transferred to the fluorescent-emitting material in the fluorescent-emitting layer. In view of this, it has been proposed to provide a space layer (occasionally also referred to as a blocking layer, a bipolar layer, an interlayer or the like) between the fluorescent-emitting layer and the phosphorescent-emitting layer.
For instance, an organic EL device disclosed in Patent Literature 1 includes a bipolar layer, through which holes and electrons can be transferred, between an emitting layer containing a fluorescent dopant material and an emitting layer containing a phosphorescent dopant material. The bipolar layer contains a compound having triplet energy larger than triplet energy of the phosphorescent dopant material. In Patent Literature 1, such a device arrangement enables to provide an organic EL device that inhibits Dexter transfer of triplet energy while maintaining carrier balance between the emitting layer containing the fluorescent dopant material and the emitting layer containing the phosphorescent dopant material, and exhibits excellent luminous efficiency and whiteness index.